Connected: An Internet Encyclopedia
RFC 950 - 3. A Class C Network Case (illustrating non-contiguous subnet bits)

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### 3. A Class C Network Case (illustrating non-contiguous subnet bits)

For this case, assume that the requesting host is on class C network 192.1.127.0, has address 192.1.127.19, that there is a gateway at 192.1.127.50, and that on network an 3-bit subnet field is in use (01011000), that is, the address mask is 255.255.255.88.

The host sends the ICMP request to 255.255.255.255:

             Source address:          192.1.127.19
             Destination address:     255.255.255.255
             Protocol:                ICMP = 1
             Type:                    Address Mask Request = AM1
             Code:                    0
             Mask:                    0

The gateway can then respond directly to the requesting host.

             Source address:          192.1.127.50
             Destination address:     192.1.127.19
             Protocol:                ICMP = 1
             Type:                    Address Mask Reply = AM2
             Code:                    0
             Mask:                    255.255.255.88.

In the diskless workstation case the host sends:

             Source address:          0.0.0.0
             Destination address:     255.255.255.255
             Protocol:                ICMP = 1
             Type:                    Address Mask Request = AM1
             Code:                    0
             Mask:                    0

192.1.127.50 will hear the datagram, and should respond with this datagram:

             Source address:          192.1.127.50
             Destination address:     255.255.255.255
             Protocol:                ICMP = 1
             Type:                    Address Mask Reply = AM2
             Code:                    0
             Mask:                    255.255.255.88.

If broadcasting is not allowed 192.1.127.19 sends:

             Source address:          192.1.127.19
             Destination address:     192.1.127.50
             Protocol:                ICMP = 1
             Type:                    Address Mask Request = AM1
             Code:                    0
             Mask:                    0

192.1.127.50 should respond exactly as in the previous case.

             Source address:          192.1.127.50
             Destination address:     192.1.127.19
             Protocol:                ICMP = 1
             Type:                    Address Mask Reply = AM2
             Code:                    0
             Mask:                    255.255.255.88

Next: Appendix III. Glossary

Connected: An Internet Encyclopedia
RFC 950 - 3. A Class C Network Case (illustrating non-contiguous subnet bits)